1. Field of the Invention
The present invention relates generally to roofing structures and, more particularly, to ballast blocks designed for use in single ply roofing construction. Specifically, the present invention relates to an improved ballast block construction wherein the ends of such ballast blocks may be interlinked to prevent uplift and rotational displacement due to wind forces.
2. Description of the Prior Art
For many years, flat roofs generally found on commercial structures have traditionally utilized built-up roofing technology. The standard composite-type roof is an example of such technology. However, single-ply membrane roofing has been growing in popularity over the years and has captured a substantial portion of the flat roof market. The significant advantages of single-ply roofing are that such structures are easier to install, more reliable and economical, and much more durable.
In single-ply roofing construction, a waterproof single-ply membrane is laid over a layer of insulation, and both layers are held in place by a ballast system. The single-ply membrane is commonly made from rubber, plastic or some other type of waterproof material. Of the various techniques commonly used for installing the single-ply membranes, the most popular is loose laid, either over or under the roof insulation. These materials are then held in place by the ballast.
Conventionally, there are four basic types of ballast systems presently in use. The first of these includes a layer of loose laid, well-rounded stones having diameters generally ranging from 3/4 to 11/2 inches and applied at a design rate of about 10 pounds per square foot. Second, standard paving blocks can be used having a nominal thickness of 17/8 inches to 21/4 inches with a unit weight of 18-25 pounds per square foot. A third type includes a composite tongue and groove board made with a layer of heavy concrete bonded to an extruded polystyrene insulation and having a unit weight of about 5-6 pounds per square foot. Finally, lighter weight ballast blocks which are specifically designed for single-ply roofing structures have also been utilized.
Each of the above ballast systems has been applied in a variety of circumstances. Criteria for ballast systems as developed by building codes, insurance requirements and various manufacturers, indicate that ballast for single-ply, loose laid membranes must be placed in such a fashion that the total coverage of the waterproof membrane is obtained while satisfying four basic conditions. The ballast must adequately protect the membrane from uplift forces developed from naturally occurring winds. The ballast system must provide adequate coverage to prevent flame spread and damage from flying hot embers. The ballast must also protect the membrane layer from the deleterious effects of ultraviolet rays from the sun. Finally, the ballast must provide a layer which protects the membrane from puncturing, tearing and the like.
Failure of ballasted roof systems generally occurs when a sufficient amount of the ballast material actually moves out of position on the roof thereby exposing the underlying insulation or membrane to the direct action of wind and/or sunlight. This can cause substantial damage since the membrane may degenerate due to sun exposure or be ripped and blown off the roof by the wind. It has been documented that loose laid stone will vibrate, scour, and even become airborne under certain wind conditions. Thus, ballast systems incorporating loose stone have not generally provided an adequate roofing structure over a prolonged period of time, although it has been one of the more popular systems due to its ease of installation. It has also been found that conventional ballast blocks may be subject to uplift from wind forces as well as freezing of ponded water. This uplift can cause rotation of the blocks and thereby expose the underlying membrane to the environment. Moreover, the uplift of ballast blocks can puncture and tear the underlying membrane material as a result of the abrasive effect of the block against the membrane. Thus, there is still a need for a ballast system or structure which is designed to provide adequate ballast, proper drainage, a walking surface across the roof which prevents puncturing of the membrane therebelow, as well as a structure which is resistant to wind uplift forces including substantial wind forces of 80 mph or above.